1. Field of the Invention
The present invention relates to a method of trimming thin film resistors, and to integrated circuits including the same.
2. Background of the Invention
Resistors can be fabricated within integrated circuits. Although resistors on a single integrated circuit may be matched with respect to each other, process variations within the fabrication process can result in the resistances of the resistors varying by significant amounts between integrated circuits or between design targets and fabricated values, such as a variation of up to about 20%. To calibrate such integrated resistors, which are normally provided as thin film resistors, methods such as a laser trimming and provision of additional resistors with fusible links can be used. Laser trimming has been successful in obtaining the degree of calibration required, but can only be carried out prior to assembly of the integrated circuit in a package. Laser trimming cannot be used to modify the power coefficient of resistance (“PCR”) of a resistor.
Fusible link trimming can be used to provide additional resistors that are fabricated in association with respective fusible links, which can be in series or parallel with a resistor, and which can be selectively blown by application of a relatively large current to trim out resistor values to a desired value. Semi-fusible links can be provided, which in their “blown” state have a higher resistance value but are not open circuit. Such links are often associated with active programming circuitry in order that they can be selected for blowing. However these techniques do not allow the PCR of the resistor to be controlled.
The PCR is the change in resistance value as a function of the power dissipated by the resistor. The dissipated power can be determined by the product of the current through the resistor and the voltage across it. There is a need for a method of trimming resistors which can be used to control or modify the PCR of the resistors.